Check out this cool DIY on how to build your own 3D motion sensing controller.
Make a 3D interface using an six resistors, aluminum foil, and an Arduino. Take that, Wii.
The basic goal here was to make a 3D position sensing system that most people can build, while still preserving some semblance of functionality. To get an idea of possible applications, check out the demo video. If you think you can build one that is simpler and equally accurate, or slightly more complex and more accurate, share in the comments!